Field of the Invention
The invention relates to a method for plasma-chemical production of nitrogen monoxide (NO), in particular for medical purposes. It also relates to an apparatus for plasma-chemical production of NO.
Nitrogen monoxide is a biologically important molecule for intracellular and intercellular transmission of impulses and for immunological reactions, which is normally also formed in the human body. The relief of stress in blood vessels is influenced by NO in the same way as the relief of stress in the alveoli or the smooth musculature in the gastric region. Conversely, a lack of NO can lead to the relaxation of the smooth musculature being suppressed, and thus to constrictions occurring. Such a constriction is evident, for example, in the bronchial area in the form of asthma. Many of those symptoms can be overcome, as is known, by inhalation of gas mixtures containing NO, with NO concentrations in the range of 0.5 ppm to 200 ppm.
Until now, such an inhalation gas containing NO has been obtained from gas cylinders, the storage and handling of which in a clinic or some other therapy facility is complex due to the safety measures required. That applies in particular to a mobile apparatus. Furthermore, the quality of the stored gas has to comply with stringent requirements for medical applications. Those requirements further increase the cost for production and storage. Specifically, even a minor impurity in the gas leads to the formation of undesirable, and possibly toxic, byproducts.
International Publication No. WO95/07610 discloses a method for plasma-chemical production of NO, in which NO is produced under the influence of a corona discharge in a process gas containing nitrogen (N.sub.2) and oxygen (O.sub.2). The corona discharge is operated continuously. A gas discharge of the described type disadvantageously leads to only a comparatively minor heating of the process gas to a temperature which is on the order of magnitude of 200.degree. C. That comparatively low temperature only allows NO to be produced in a gas mixture with low efficiency. In fact, the NO.sub.2 (which is undesirable for inhalation purposes) is preferentially produced. In order to remove the NO.sub.2 from the inhalation gas, it is necessary to use a costly absorber technique. The disadvantage of an absorber is, in particular, that the absorber material must be replaced or reconditioned frequently.
A further method of that type is disclosed in U.S. Pat. No. 5,396,882. In that case, the plasma is produced through the use of a spark discharge (or else an arc discharge) instead of by the corona discharge. The spark discharge is highly energetic in comparison to a corona discharge and produces a comparatively large amount of gas heating, as a result of which correspondingly efficient NO production is achieved. However, the high thermal load on the electrodes, particularly at the spark contact point, disadvantageously leads to severe electrode erosion, that is to say to progressive decomposition of the electrode material. Due to that electrode erosion, the known method on one hand requires intensive maintenance since the electrodes are highly susceptible to wear. On the other hand, it is necessary to prevent the eroded electrode material, which is finely distributed in the inhalation gas, from reaching a patient's breathing passages. That requires complex cleaning of the inhalation gas.
The prior art also discloses other non-thermal gas discharges, for example dielectric barrier discharges, as other alternatives to a corona discharge or spark discharge. For example, German Patent No. 438309 describes a device for carrying out chemical reactions with the aid of high-voltage currents using outer electrodes which are covered with semiconductors, and a thin wire as an inner electrode that is also referred to as a corona electrode, in the case of which additives to achieve a catalytic effect are also added to the reaction mixture. However, that device is not suitable for producing pure nitrogen monoxide. That is because corona electrodes operate at low current densities and are therefore unsuitable for injecting sufficiently high electrical power levels for NO production, and the addition of a catalyst to the reaction gas mixture must, in principle, be regarded with apprehension for medical applications.